The present invention relates generally to optical transport networks and particularly to improving the performance of forward error correction (FEC) in such networks.
As increasing demands are made on the world's communications networks, new standards emerge to cater for the challenges. The optical transport network (OTN) was developed in order to provide the transmission needs of today's wide range of digital services requiring significant transmission bandwidth and speed. OTN was conceived in 2001 to overcome the drawbacks of current optical networks such as the synchronous optical network (SONET) or the synchronous digital hierarchy (SDH). The OTN capabilities and facilities were published as a standard, known as ITU—G.709 “Network node interface for the optical transport network (OTN)” (hereinafter “G.709 standard”). The G.709 standard is based on the definitions for SONET and SDH with some additional key elements targeted towards improving the performance and reducing the cost. These include management of optical channels in the optical domain, forward error correction (FEC) to improve error performance and enable longer optical spans, and a standardized method for managing optical wavelengths (channels) end-to-end without the need for processing of the payload signal.
Reference is now made to FIG. 1, which shows an illustration of an OTN frame structure 100. An OTN frame consists of three distinct parts: an overhead area 110, a payload area 120, and a forward error correction (FEC) area 130. Overhead area 110 includes data for operation, maintenance functions, and administration. Payload area 120 includes consumer data to be transported. FEC area 130 is used to improve the error avoidance performance (both detection and correction), which further enables the placement of longer optical spans.
FEC has been used in telecommunications for many years, mainly in the areas of satellite communications and undersea data transport. FEC has been important in enabling communications to maintain acceptable performance quality in noisy environments, while keeping infrastructure costs within reason. As transmission bit rates increase to 10 Giga bits per second (Gbps) and above, the physical parameters of the optical fiber network play a more significant role in the degradation of transmitted pulses of light. FEC provides additional coded data to enable error checking and correction by a receiving device. The G.709 standard includes a standard FEC that enables long haul transmission at higher line rates without degraded performance.
The FEC method used in the OTN is a Reed-Solomon RS (255,239) code. This means that for every 239 bytes of data, another 16 bytes of data are added for the purpose of error correction. Using the Reed-Solomon scheme for FEC, eight error symbols can be corrected, and sixteen error symbols can be detected.
Reference is now made to FIG. 2, which shows a schematic diagram illustrating the method for creating FEC data by processing overhead area 110 and payload area 120. In order to create FEC area 130, i.e., the RS (255,239) code, each row of overhead area 110 and payload area 120 is divided into 239 groups 210-1 through 210-239. Each group 210 includes sixteen consecutive bytes belonging to overhead area 110 and payload area 120. As can be seen in FIG. 2, group 210-1 includes the first sixteen bytes, i.e., bytes “1” through “16” of OTN frame 100, group 210-2 includes the next sixteen bytes, i.e., bytes “17” through “32” of OTN frame 100, and so on. For each row, additional sixteen groups 220-1 through 220-16 of sixteen bytes each are added to the groups 210. Then groups 210 and 220 are passed through a Reed-Solomon encoder, which produces the FEC code words. The process is repeated for the other three rows, thus handling the entire OTN frame. In an OTN frame, each row contains 16 FEC code-words of 16 bytes for the row, resulting in 64 FEC code-words (4×16) for every OTN frame.
A Bose-Chaudhuri-Hocquenghem (BCH) code is an example of a code that can be used for correcting errors in input data. The BCH code is used in satellite communication links, where error correction can be employed to mitigate the effects of noise interference. The BCH code has been widely used in practice due to its high flexibility in choosing code length and number of correctable errors, as well as its error detecting capabilities, which are achieved using relatively few check bits. The BCH code, however, requires complex decoding algorithms to reconstruct the information from a received signal. The complex decoding algorithms have typically been implemented by special-purpose computers, which perform computations in real time.
As the need for very high-speed encoders and decoders has developed, the limitation of the computation technology has become apparent. Even with the most sophisticated high-speed digital logic circuits, the highest achievable data rate appears to be about 1 Gbps.
A conventional BCH decoder corrects the errors by applying the following four steps: (1) calculating the syndromes; (2) calculating the error location polynomial; (3) calculating the error location numbers; and (4) correcting the errors. The syndromes are the values that contain the information needed to identify and locate any errors. The syndromes are usually computed by dividing the received data with a generator polynomial. The conventional technique for translating the syndrome patterns into the error location polynomial is performed using the Berlekamp algorithm, disclosed in U.S. Pat. No. 4,162,480 and 4,410,989. The error location polynomial is solved by means of the Chien search method, disclosed in U.S. Pat. No. 5,974,582.
The use of RS code in OTN bounds the number of errors that can be corrected to an upper limit. Hence, in order to improve the error correction performance in OTN, there is a need to replace the existing RS code with an alternative code that can ensure an improved error correction. State of the art BCH encoders and decoders are not capable of encoding and decoding information in excess of 10 Gpbs. Furthermore, there is a need to adapt the OTN frame structure to handle the BCH code, i.e., to replace the RS code with the BCH code. Therefore, it would be advantageous to provide an apparatus and method for processing OTN frames while performing error correction by means of a BCH code. It would be further advantageous to provide a BCH decoder capable of decoding information at rates of 10 Gpbs and upwards, and a BCH encoder capable of manipulating the OTN frame to include the BCH code-words.